Offset lithographic printing has been based for many years on the use of imaged plates, where background non-printing areas are covered during the printing process with aqueous fount solution and the print areas on the plate are inked up with oleophilic inks which provide the printed matter on the paper or other types of substrate upon which the print is required. The aqueous fount provides an oleophobic surface to prevent inking of the non-image areas. Thus, the offset lithographic plate must be imaged in such a way as to provide areas which are hydrophilic and can be covered with fount (corresponding to areas of background on the final print) and areas which will not accept the fount and are therefore hydrophobic, which will then receive the ink for printing. The offset printing machine contains means of continuously supplying both ink and fount in order to produce multiple printing impressions. The supplies of ink and fount must be carefully controlled and balanced to produce good quality prints with no ink in the background.
In U.S. Pat. No. 3,511,178 Curtin described waterless printing, where instead of relying on the water to repel the ink, the background areas are coated with an oleophobic layer, so eliminating the need for fount and making control of the press easier. The material most widely used for the oleophobic layer has been polydimethyl siloxane (PDMS).
As described in the Curtin patent and in many subsequent patents on waterless printing, the imaging process involves selective removal of the silicone coating. The layer uncovered by the imaging process is ink receptive. The ink receptive layer is usually based on a polymeric layer. In an offset press, the ink supplied to the printing plate by the inking system will be rejected by the silicone layer and accepted by the areas where the silicone was removed.
In order to image the plate, the silicone can be selectively removed by methods such as spark erosion, thermal ablation and selective curing of layers underneath the silicone film, to alter the adhesion of the top silicone coating to the undercoat. Instances of these processes may be found in, GB 1,490,732, and U.S. Pat. Nos. 6,004,723 and 3,511,178. In the latter case, a chemical development process removes the PDMS from the uncured areas of the under-layer.
Apart from the properties of ink reception and ink repulsion, there are other important properties that the plate should have. Examples of these properties are high imaging sensitivity, good shelf life, sufficient robustness to withstand printing multiple impressions and chemical resistance to ink and cleaning materials. The full functionality of the plate is achieved by the use of materials which provide specific properties. For instance, chromophors or other materials which absorb radiation are used in the case of Ultra Violet (UV) or infrared (IR) imaging to absorb the appropriate radiant energy which is then used to form the selective image. Polymeric materials are used to bind the radiation materials and to provide bonding between the PDMS layer and the substrate.
It is important to emphasize that the positioning of the materials in the plate structure plays a major role in the plate performance. Most waterless plates have multiple coatings where, for example, the topcoat is oleophobic and does not include ink receptive materials; these are usually located in one of the under-layers. In plates imaged using thermal ablation, electromagnetic energy is transformed into thermal energy by absorption into material embedded in the plate. It is usual for a thermally insulating layer to be positioned between the substrate and the ablating layer if the substrate is highly thermally conductive, as for instance with aluminum. This reduces the dissipation of the thermal energy produced during imaging; such dissipation would make the plate less thermally sensitive. A thermally absorbing material which is instrumental in producing the image is usually located in one of the under-layers. In order to best achieve performance, multiplayer systems have been devised. U.S. Pat. No. 6,045,964 to Ellis, et al. provides an example of a waterless plate on an aluminum base utilizing 5 separate layers.
Thus, to meet the above requirements for optimal functionality of the printing plate, the accepted solution is layered structures, where different layers contribute different properties required for the functionality of the plate. The layered structure does not modify the silicone layer surface, and allocates the different materials to their appropriate places in the plate.
In industrial manufacturing, each of the layers of the offset printing plate is coated separately on a suitable coating line. Occupying a coating line is expensive. A significant part of the process of setting-up the coating machine and reaching constant coating conditions has to be done whilst running a web substrate and applying the coating materials. As a result, a significant amount of substrate, as well as coating material, is wasted. The more layers applied, the more material is wasted. Note that the waste is becoming more expensive the more layers are applied. In addition, adhesion between the coated layers is always an issue to be concerned about.
Several inventors have suggested incorporating all of the required materials for the formation of a waterless offset plate into one layer, where the substrate of the printing plate, polyester for example, serves as the ink-accepting layer. Nechiporenko and Markova in a paper published in 1979, “Direct Method of Producing Waterless Offset Plates by Controlled Laser Beam” “Preprint 15” International larigi Conference 1979, warned of the danger of attempting to incorporate dyes, pigments or other such materials into the top layer of the plate, as they found that it adversely affected the oleophobic properties of the silicone layer. Nevertheless, Landsman, in U.S. Pat. No. 6,477,955, claims a one-coat ablatable waterless plate and Lewis, in U.S. Pat. No. 5,339,737, describes a one-coat silicone layer waterless plate. No details of the press performance of such constructions are given and it would be likely from the comments of Nechiporenko et al. that this would be extremely limited.
U.S. Pat. No. 6,218,780 to Ben Horin et al. describes a one-coat system primarily for use on-press for a plateless application. This was based on a silicone emulsion where the infrared absorbing material is dispersed or dissolved in the aqueous phase. Such emulsions necessitate the use of low molecular weight polydimethyl siloxanes, which have limited robustness properties and consequently are only described for press run lengths of 5000 impressions. Although this may be sufficient for some applications, it does show a limitation that would indicate the limits of robustness.